MetroSound AMS 400 Modifications
Fellow AM STEREO fanatic Darwin Long has come up with some modifications for the MetroSound AMS AM STEREO Converterthat will allow you fine tune it for maximum performance. Darwin has also included instructions on how to construct a simple yet extremely effective antenna to turn the MetroSound AMS 400 (or any radio) into a DX machine.
My sincere thanks to Darwin for passing this info along!
I have two MetroSound tuners, one in my Toyota RAV4 which fits perfectly in the compartment below the radio deck.
I had a problem with the tuning light/stereo decoder activating while tuned off-center from a station, but it can be fixed. First,hook up the tuner to the power supply and an antenna, and make sure you're able to hear it through some audio source. You'll need a small flat-blade screwdriver, preferably a non-inductive plastic or TV repair type. Next, remove the cover. Tune in a station to the desired middle position (where you want the light to illuminate). Now locate L9, towards the left center of the PC board near the slug tuner module. Adjust this transformer until the light illuminates. Now the MetroSound's tuning light and stereo decoder will activate while tuned to that middle position of any station.
You may notice that the MetroSound's tuning light and stereo decoder only activates while a strong station is tuned in. This can be adjusted so that even very weak stations can trigger the tuning indicator/stereo decoder system. Find the potentiometer VR2, located on the right side of the PC board, next to the 3-pin wiring harness. Using a small screwdriver, rotate this potentiometer to the left all of the way until it stops. Now the MetroSound will respond to even weak stations by illuminating the tuning light and decoding stereo when they are properly tuned in.
The MetroSound can also be easily adjusted so it tunes from 520-1710 kHz. First, tune the MetroSound all of the way to the bottom of the dial. Using a signal generator, known signal, or even an adjacent station at 530 or 540 kHz to judge the position of 520 kHz, adjust L6 until the tuner is receiving at 520 kHz. Next, tune the MetroSound all of the way to the top of the dial. Using a signal generator, known signal, or adjacent station below 1710 to judge the position of 1710 kHz, adjust the trimmer capacitor located directly to the right of L6 so that the tuner is receiving at 1710 kHz. Now, tune the tuner to a weak station around 600 kHz. Adjust L7 (right next to L6) so that the signal is received as strong as possible (Note: the station may shift up or down in frequency very slightly as L7 is adjusted, so "retune" it with the tuning knob). Finally, tune the MetroSound to a weak station around 1500 kHz. Adjust the trimmer capacitor located directly to the right of L7 so that the signal is received as strong as possible. Your MetroSound now receives the full 520-1710 kHz AM band. Go back and adjust L9 to make sure that the tuning light and stereo decoder activate while tuned to the center position of the stations.
Make sure that when you have the MetroSound connected to the actual antenna it will be utilizing, you adjust the antenna trimmer, too (while tuned around 1450 kHz).
The output of the jack in the back is not equalized and is excessively bright, unless an equalizer or tone controls are used. I use a CD-to-Cassette adapter plugged into the back of the MetroSound and the cassette placed into my car radio tape deck to play like a tape. Works great, but I had to put some resistors and capacitors in the MetroSound's output to equalize the output to the cassette adapter to prevent overdrive and distortion both in the cassette deck, and in the MetroSound's audio amp (overloading the outputs with a low impedance). I can supply the values and a schematic if anyone is interested in using the MetroSound this way. It approximates what the AMAX spectrum should be, though this is an older tuner. One added benefit I found is that I can use the Dolby NR on the car tape player, yielding noise reduction for Dolby AM STEREO! Sounds fantastic on a strong stations, and reduces or eliminates noise on weak signals, too!
I also had two problems with using the FM transmitter frequency to listen to the MetroSound on my radio. First, the transmitter frequency drifted significantly with temperature, making it difficult to use when your digital FM tuner has odd-numbered 200 kHz increments. Second, there was a heterodyne on many AM frequencies due to the FM pilot clock crystal harmonics. I recommend clipping a lead on that crystal if it bothers you and you're using the MetroSound hard-wired to an audio input or cassette adapter.
Hope these hints help maximize your MetroSound's performance!
I can get 1680 WJNZ in Grand Rapids, Michigan here in Southern California *IN STEREO* when I use my MetroSound in conjunction with a 200 foot longwire, earth ground, and a coupling transformer with both windings on either side tuned to match the frequency. With the MetroSound connected and headphones plugged in, great separation can be heard. It is bar none the best and most selective BCB stereo AM DX setup I have used thus far.
I highly recommend anyone needing an antenna that delivers results instead of noise to build this tunable air-core transformer system connected to an outdoor longwire of at least 100 feet. It can be used with any radio with an antenna input, or even one without (just set the radio near the coil).
The transformer is very easy to make. Simply get a toilet paper tube and also a larger diameter plastic bottle or container (about 5" diameter with one end cut off) and wrap about 80 turns of #24 wire around each, tapping about every 5 turns on each. Then take alligator clips and clip your longwire antenna to one end of a tuning capacitor. Clip the other side of the tuning capacitor to one end of the large coil. Clip the earth ground to one of the taps. Now plug a Motorola plug with cable into the back of the Metrosound (or any radio) and put alligator clips on the ground and the antenna terminals.
Clip the center conductor antenna input to a tuning capacitor, and the other side of the tuning capacitor to one end of the smaller coil. Clip the radio's chassis ground to a tap on the small coil.
Lay the coils end-to-end about 6 inches apart. Close the plates on both tuning capacitors. Select a station on the radio. Now, try choosing different taps on the large coil - the bigger the coil, the lower the frequency. When you find a tap that gives the best signal, fine-tune with the tuning capacitor on the big coil for highest signal. Then go to the small coil and try different taps for highest signal - again, the longer the coil, the lower the frequency. Again, when a tap is found with the best signal, fine-tune with the capacitor. Now, you can move the coils apart or together to adjust gain, and slide the small coil into the center of the big coil for maximum transfer.
If you change frequencies, you will need to re-adjust both sides of the transformer to peak the gain and selectivity on that frequency.
For radios with only internal loops, you can set the radio with its ferrite bar in line with the big coil and simply adjust the big coil, and the signal will be inducted into the internal antenna for super receiving ability without a direct connection. It's basically a Select-A-Tenna with a longwire attached.
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